Problem
You climb the last few rungs of the ladder and step inside. “Wow, is this cool!” Your friend is right behind you. “I told you so.”
It’s your first time in your friend’s tree house, and what a sweet setup. It has just about everything you could want in a backyard hideaway—two comfy chairs, a table, a bookshelf, games, even a TV.
You plop down in a chair and look around. Then yourealize something. “I’m hungry. Got any food up here?” “Oh, I knew I forgot something,” your friend says.
Just then, your friend’s grandmother comes out of the house carrying two lunch bags.
“Yay, Grandma, you read our minds! Now, if only there was a way to get the bags up here.”
“Hmm . . .” Grandma stares at a thick branch near the doorway of the tree house. “I have an idea. I’ll be right back.”
Solution
A few minutes later, Grandma comes back with a rope and a bucket. “What’s that for?” your friend asks. “You’ll see,” Grandma says as she ties a big knot in one end of the rope.
She swings the knotted end a few times and lets it fly. The rope sails over the branch. The knot dangles just a few feet above the ground. Grandma undoes the knot and ties the end to the bucket handle.
“Oh, I get it!” you say. “What a great idea!” Grandma places the lunch bags in the bucket and pulls the other end of the rope. With each pull, the bucket rises closer to the tree house door. At the top, it’s easy to reach out and grab the bucket . . . and lunch. Now, the tree house is perfect. It even has a food delivery system!
It's Simple
How was Grandma able to deliver food to you and your friend without climbing the ladder? The answer is “simple.” She used the rope and the branch as a simple machine. Like all machines, simple machines help us do jobs, usually by moving things faster, farther, or in an easier way.
Simple machines have no more than a few parts. In fact, some are made of only one part. They can be combined in endless ways to accomplish tasks. But how can a rope and a branch be a machine?
In the Groove
Well, based on the headline of this article, you probably know the rope and branch formed a pulley. A pulley is a wheel with a cord wrapped around it. The cord might be a string, a rope, a chain, or even a steel cable. Usually the cord fits in a groove around the rim of the wheel. That way, the cord doesn’t slip out. The wheel turns freely as the cord is pulled through the groove.
The branch doesn’t have a groove, and it certainly doesn’t turn like a wheel, but the idea is the same. When Grandma pulled down on one end of the rope, the other end, with the bucket attached, moved up. That’s one way a pulley makes it easier to lift objects. It changes the direction of a force.
A flagpole works the same way. A rope is looped through a pulley at the top of the pole. To raise the flag, you attach it to one side of the rope and pull down on the other side. The flag goes up!
Like any simple machine, a pulley doesn’t do the work for you. To raise the flag, you have to pull with a certain amount of force. The pulley doesn’t increase your force. It just changes its direction. But that’s a lot easier than climbing up the pole, tying the flag at the top, and then sliding down again.
How to raise a flag with a pulley
1
Pull down on this end of the rope.
2
The rope moves through the pulley.
3
The flag moves up with this end of the rope.
